PKWARE MEMORY CHIP DRIVERS
Other components, such as device drivers and TSRs, could be loaded into the upper memory area (UMA). Starting with DR DOS 5.0 (1990) and MS-DOS 5.0 (1991), parts of the operating system could be loaded into the HMA as well, freeing up to 46 KB of conventional memory. The first user of the HMA among Microsoft products was Windows/286 2.1 in 1988, which introduced the HIMEM.SYS device driver. So-called A20 handlers could control the addressing mode dynamically, thereby allowing programs to load themselves into the 1024–1088 KB region and run in real mode. This gate could be controlled, initially through the keyboard controller, to allow running programs which wanted to access the entire RAM. This circuit was a simple logic gate which could disconnect the microprocessor's 21st addressing line, A20, from the rest of the motherboard. To allow running existing DOS programs which relied on this feature to access low memory on their newer IBM PC AT computers, IBM added special circuitry on the motherboard to simulate the wrapping around. (Does this make sense? I'll explain later) The Intel 8086 and Intel 8088 processors, with only 1 MB of memory and only 20 address lines, wrapped around at the 20th bit, so that address FFFF:0010 was equivalent to 0000:0000. Memory as part of the 64 KB range starting 16 bytes before the 1 MB So this mode can actually address the first 65520 bytes of extended Although intended to address only 1 Megabyte (MB) (2 20 bytes) of memory, segment:offset addresses at FFFF:0010 and beyond reference memory beyond 1 MB ( FFFF0 + 0010 = 100000). Segment and a 16-bit offset, which is resolved into a physical address In real mode, the segmentation architecture of the Intel 80286Īnd subsequent processors identifies memory locations with a 16-bit In DOS memory management, the high memory area ( HMA) is the RAM area consisting of the first 65520 bytes above the one megabyte in an IBM AT or compatible computer.
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